It is well known to use electromagnetic radiation to detect the presence of objects (e.g. handheld detectors used for detecting objects on or under the ground, and walk-through arches at airports). However, the conventional detectors used at airports may be unable to determine the dimensions of objects to any significant degree, and thus may be unable to distinguish between objects of different types, i.e. harmless (belt buckles, cameras), and potentially dangerous (guns, knives).
The use of microwaves (electromagnetic waves with wavelengths in the centimetre to millimetre range) may provide a means for the standoff detection and identification of concealed conducting items such as handguns and knives. Large metal objects, such as handguns, may give a significantly different and generally larger response when irradiated by low power microwaves than the human body, clothing and/or benign normally-carried objects. The larger response may be detected using a combination of antenna and receiver. When an object is illuminated with swept and/or stepped frequency microwave radiation, the frequency response of radiation returned from the object may provide information regarding dimensions of the object.
It is desirable to provide a system which is capable of detecting objects, for example knives, in a manner which is not disclosed in the prior art.